/Engine/Source/Runtime/Core/Public/Math/Matrix.h
Transform a direction vector - will not take into account translation part of the TMatrix. If you want to transform a surface normal (or plane) and correctly account for non-uniform scaling you should use TransformByUsingAdjointT.
[TVector4](API\Runtime\Core\Math\TVector4)< T > TransformVector ( const [TVector](API\Runtime\Core\Math\TVector)< T > & V ) const